1. Field of the Invention
The present invention relates to a sheet containing apparatus and an image forming apparatus having such a sheet containing apparatus.
2. Related Background Art
In the past, in image forming apparatuses such as copying machines, printers and facsimiles, a sheet feeding apparatus for feeding a sheet to an image forming portion was provided and a sheet containing apparatus for containing sheets to be fed was provided in the sheet feeding apparatus.
As an example of such a sheet containing apparatus, there is a sheet feeding cassette detachably attached to an image forming apparatus. Among such cassette, there is a cassette of so-called separation claw type having a separation claw as a separation system for separating and feeding out sheets one by one. Since the separation claw system can separate and feed out the sheets one by one with a relatively cheap arrangement, it is widely used in the sheet feeding cassette.
Further, as one of such sheet feeding cassettes, a universal cassette capable of containing sheets having various different sizes is known. In such a universal cassette, a side regulating plate and a trailing end regulating plate which can be shifted to positions corresponding to various sheet sizes are provided within a body of the sheet feeding cassette, so that sheets having various sizes can be contained in the cassette by changing the positions of the side regulating plate and the trailing end regulating plate.
Incidentally, particularly when the sheet feeding cassette is compact or thin, the side regulating plate is often formed from resin such as ABS or a thin sheet metal. In this case, an abutment face for regulating a lateral edge of the sheet is a reference face for lateral edges of the sheets to be contained and has an important role for regulating skew feeding and lateral registration of the sheet.
FIG. 11 is a top plan view of a conventional universal cassette, FIG. 12 is a sectional view taken along the line XII--XII, and FIG. 13 is a sectional view taken along the line XIII--XIII.
In FIGS. 11 and 13, a universal cassette (referred to as "sheet feeding cassette" hereinafter) 1 includes an intermediate plate 2 rotatable around a shaft 3 secured to the sheet feeding cassette 1, an intermediate plate spring 13 for lifting the intermediate plate 2, a sheet feeding roller 21 rotatable in a direction shown by the arrow, sheets S such as transferring materials, and a trailing end regulating plate 7 for regulating trailing ends of the sheets.
Further, the sheet feeding cassette 1 is provided with a pair of side regulating plates 5, 6 movable in a width-wise direction of the sheet S and adapted to regulate left and right lateral edges of the sheets S, and the side regulating plates 5, 6 has racks 5a, 6a, respectively, and a pinion gear 8 is engaged by central portions of the racks 5a, 6a.
By the provision of such a rack mechanism, the front side regulating plate (referred to as "first side regulating plate" hereinafter) 5 and the rear side regulating plate (referred to as "second side regulating plate" hereinafter) 6 in a mounting direction of the sheet feeding cassette 1 can be slid in a direction shown by the arrow D.sub.11 in synchronous with each other so that both lateral edges of the sheets S can be regulated simultaneously with respect the center as a conveyance reference of sheet. Incidentally, FIGS. 11 and 13 show a condition that sheets S having minimum size are contained in the sheet feeding cassette 1.
After both lateral edges of the sheets S having minimum size contained in the sheet feeding cassette 1 are regulated, as shown in FIG. 14, a fixing member 14 of the first side regulating plate 5 is rotated to a position shown by the broken line.
By rotating the fixing member 14 in this way, a rib leading end 14a of the fixing member 14 is inserted into a fixing groove la corresponding to the minimum size sheet S among a plurality of fixing grooves provided in the sheet feeding cassette 1 in correspondence to various sheet sizes, with the result that the side regulating plates 5, 6 are fixed.
Incidentally, in FIG. 14, a pair of conveying rollers 23 serve to conveying the sheet S fed out by the sheet feeding roller 21.
The side regulating plates 5, 6 have separation claws 11, 12, respectively. Similar to a well-known separation claw system, the separation claws 11, 12 catch both front corners of the sheet S, and overlapping widths between the separation claws 11, 12 and the sheet S are normally set to be constant regardless of the sheet size.
In the sheet feeding apparatus having the above-mentioned arrangement, when a sheet feeding signal is received from a main body of the image forming apparatus, for example as shown in FIG. 13, the semi-circular sheet feeding roller 21 is rotated in the direction shown by the arrow. As a result, when the roller is urged against the uppermost sheet S, the sheets S are fed and then are caught by the separation claws 11, 12.
Incidentally, the sheets S caught by the separation claws 11, 12 are further fed by the sheet feeding roller 21 in a condition that the leading ends of the sheets are regulated, with the result that loops are formed in the sheets between the sheet feeding roller 21 and the separation claws 11, 12.
As the loops grow, a repulsion force is also increased. Consequently, sheets other than the uppermost sheet forcibly looped by the sheet feeding roller 21 cannot maintain their loops due to their own repelling forces, with the result that these sheets are separated from the uppermost sheet. On the other hand, since the separation claws 11, 12 regulate only the front corners of the sheets, when the repulsion force of the looped uppermost sheet exceeds the regulating force of the separation claws 11, 12, the front corners of the uppermost sheet rides over the separation claws 11, 12, and the separated uppermost sheet is conveyed by the pair of conveying rollers 23.
However, in the sheet feeding apparatus having such a sheet feeding cassette, when the mounting direction of the sheet feeding cassette is perpendicular to a sheet feeding direction, if the sheet feeding cassette is mounted with a strong force, the sheet stack is struck against the rear side or second side regulating plate 6 due to inertia. As mentioned above, since the second side regulating plate 6 is formed from resin such as ABS or thin sheet metal, if the sheet stack strikes against the second side regulating plate, as shown in FIG. 15, an upper part of the second side regulating plate will be flexed temporarily in the mounting direction.
If the upper part of the second side regulating plate is flexed in this way, as shown in FIG. 15, when it is assumed that a flexed amount of the second side regulating plate 6 is L1 and a width of the sheet S caught by the front side separation claw 11 is L, in dependence upon the force for mounting the sheet feeding cassette 1, L1&gt;L may be established, with the result that the sheet S is disengaged from the rear side separation claw 12, or, from the front side separation claw 11 if the sheet S is deviated toward the rear side due to the shock.
If the sheet S is disengaged from the separation claws 11, 12, when the sheet feeding cassette 1 is mounted or dismounted with respect to the main body of the image forming apparatus in such a condition, the sheet S is pinched between the cassette and the main body of the image forming apparatus, thereby damaging the sheet. Further, in the sheet feeding operation, since the sheet S is fed out in a condition that one front corner of the sheet S is not caught by the separation claw 11 or 12, sheet may be skew-fed and accordingly poor sheet feeding may occur.